It is highly desirable for tires to provide good treadwear, low rolling resistance and good traction on wet and dry surfaces. In order to reduce the rolling resistance of a tire, rubbers having high rebound can be utilized in making the tire tread. Tires made with such rubbers undergo less energy loss during rolling. The traditional problem associated with this approach is that wet traction and wet skid resistance are compromised. This is because good rolling resistance which favors low energy loss and good traction characteristics which favor high energy loss are viscoelastically inconsistent properties.
In order to balance these two viscoelastically inconsistent properties, mixtures of various types of synthetic and natural rubber are normally utilized in tire treads. For instance, various mixtures of styrene-butadiene rubber and polybutadiene rubber are commonly used in tire tread compounds for automobile tire treads. Blends of rubbers having a low glass transition temperature with rubbers having a high glass transition temperature offer significant advantages in treadwear and traction characteristics over low rolling resistance tread compounds made with blends of rubbers having intermediate glass transition temperatures. The major disadvantage associated with blends of rubbers having low and high glass transition temperatures is that they exhibit poor processability. Blends of high molecular weight isoprene-butadiene rubber with natural rubber can be used in formulations that offer ultra low rolling resistance to improve processability. However, such blends of rubbers having intermediate glass transition temperatures have an undesirable effect on treadwear and rolling resistance without significantly improving traction.
U.S. Pat. No. 4,843,120 discloses that tires having improved performance characteristics can be prepared by utilizing rubbery polymers having multiple glass transition temperatures as the tread rubber. These rubbery polymers having multiple glass transition temperatures exhibit a first glass transition temperature which is within the range of about −110° C. to −20° C. and exhibit a second glass transition temperature which is within the range of about −50° C. to 0° C. According to U.S. Pat. No. 4,843,120, these polymers are made by polymerizing at least one conjugated diolefin monomer in a first reaction zone at a temperature and under conditions sufficient to produce a first polymeric segment having a glass transition temperature which is between −110° C. and −20° C. and subsequently continuing said polymerization in a second reaction zone at a temperature and under conditions sufficient to produce a second polymeric segment having a glass transition temperature which is between −20° C. and 20° C. Such polymerizations are normally catalyzed with an organolithium catalyst and are normally carried out in an inert organic solvent.
U.S. Pat. No. 5,137,998 discloses a process for preparing a rubbery terpolymer of styrene, isoprene and butadiene having multiple glass transition temperatures and having an excellent combination of properties for use in making tire reads which comprises terpolymerizing styrene, isoprene and 1,3-butadiene in an organic solvent at a temperature of no more than about 40° C. in the presence of (a) at least one member selected from the group consisting of tripiperidino phosphine oxide and alkali metal alkoxides and (b) an organolithium compound.
U.S. Pat. No. 5,047,483 discloses a pneumatic tire having an outer circumferential tread where said tread is a sulfur-cured rubber composition comprised of, based on 100 parts by weight rubber (phr), (A) about 10 to about 90 parts by weight of a styrene-isoprene-butadiene terpolymer rubber (SIBR) and (B) about 70 to about 30 weight percent of cis 1,4-polyisoprene rubber or cis 1,4-polybutadiene rubber, wherein said SIBR rubber is comprised of (1) about 10 to about 35 weight percent bound styrene, (2) about 30 to about 50 weight percent bound isoprene and (3) about 30 to about 40 weight percent bound butadiene and is characterized by having a single glass transition temperature (Tg) which is in the range of about −10° C. to about −40° C. and, further, the said bound butadiene structure contains about 30 to about 40 percent 1,2-vinyl units, the said bound isoprene structure contains about 10 to about 30 percent 3,4-units and the sum of the percent 1,2-vinyl units of the bound butadiene and the percent 3,4-units of the bound isoprene is in the range of about 40 to about 70 percent.
U.S. Pat. No. 5,272,220 discloses a styrene-isoprene-butadiene rubber which is particularly valuable for use in making truck tire treads which exhibit improved rolling resistance and tread wear characteristics, said rubber being comprised of repeat units which are derived from about weight percent to about 20 weight percent styrene, from about 7 weight percent to about 35 weight percent isoprene and from about 55 weight percent to about 88 weight percent 1,3-butadiene, wherein the repeat units derived from styrene, isoprene and 1,3-butadiene are in essentially random order, wherein from about 25 percent to about 40 percent of the repeat units derived from the 1,3-butadiene are of the cis-microstructure, wherein from about 40 percent to about 60 percent of the repeat units derived from the 1,3-butadiene are of the trans-microstructure, wherein from about 5 percent to about 25 percent of the repeat units derived from the 1,3-butadiene are of the vinyl-microstructure, wherein from about 75 percent to about 90 percent of the repeat units derived from the isoprene are of the 1,4-microstructure, wherein from about 10 percent to about 25 percent of the repeat units derived from the isoprene are of the 3,4-microstructure, wherein the rubber has a glass transition temperature which is within the range of about −90° C. to about −70° C., wherein the rubber has a number average molecular weight which is within the range of about 150,000 to about 400,000, wherein the rubber has a weight average molecular weight of about 300,000 to about 800,000 and wherein the rubber has an inhomogeneity which is within the range of about 0.5 to about 1.5.
U.S. Pat. No. 5,405,927 discloses an isoprene-butadiene rubber which is reported to be particularly valuable for use in making truck tire treads, said rubber being comprised of repeat units which are derived from about 20 weight percent to about 50 weight percent isoprene and from about 50 weight percent to about 80 weight percent 1,3-butadiene, wherein the repeat units derived from isoprene and 1,3-butadiene are in essentially random order, wherein from about 3 percent to about 10 percent of the repeat units in said rubber are 1,2-polybutadiene units, wherein from about 50 percent to about 70 percent of the repeat units in said rubber are 1,4-polybutadiene units, wherein from about 1 percent to about 4 percent of the repeat units in said rubber are 3,4-polyisoprene units, wherein from about percent to about 40 percent of the repeat units in the polymer are 1,4-polyisoprene units, wherein the rubber has a glass transition temperature which is within the range of about −90° C. to about −75° C. and wherein the rubber has a Mooney viscosity which is within the range of about 55 to about 140. U.S. Pat. No. 5,405,927 specifically discloses blends of this isoprene-butadiene rubber with natural rubber or 3,4-polyisoprene.